US20130192212A1 - Exhaust purification system of internal combustion engine - Google Patents
Exhaust purification system of internal combustion engine Download PDFInfo
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- US20130192212A1 US20130192212A1 US13/259,885 US201013259885A US2013192212A1 US 20130192212 A1 US20130192212 A1 US 20130192212A1 US 201013259885 A US201013259885 A US 201013259885A US 2013192212 A1 US2013192212 A1 US 2013192212A1
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- exhaust purification
- hydrocarbons
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- exhaust
- purification catalyst
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
- F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
- F01N3/0842—Nitrogen oxides
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
- B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
- B01D53/9404—Removing only nitrogen compounds
- B01D53/9409—Nitrogen oxides
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
- F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
- F01N3/0814—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with catalytic converters, e.g. NOx absorption/storage reduction catalysts
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
- F01N3/0871—Regulation of absorbents or adsorbents, e.g. purging
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N9/00—Electrical control of exhaust gas treating apparatus
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2251/00—Reactants
- B01D2251/20—Reductants
- B01D2251/208—Hydrocarbons
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2255/00—Catalysts
- B01D2255/10—Noble metals or compounds thereof
- B01D2255/102—Platinum group metals
- B01D2255/1021—Platinum
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2255/00—Catalysts
- B01D2255/90—Physical characteristics of catalysts
- B01D2255/91—NOx-storage component incorporated in the catalyst
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N2240/00—Combination or association of two or more different exhaust treating devices, or of at least one such device with an auxiliary device, not covered by indexing codes F01N2230/00 or F01N2250/00, one of the devices being
- F01N2240/30—Combination or association of two or more different exhaust treating devices, or of at least one such device with an auxiliary device, not covered by indexing codes F01N2230/00 or F01N2250/00, one of the devices being a fuel reformer
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N2610/00—Adding substances to exhaust gases
- F01N2610/03—Adding substances to exhaust gases the substance being hydrocarbons, e.g. engine fuel
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/10—Internal combustion engine [ICE] based vehicles
- Y02T10/40—Engine management systems
Definitions
- the present invention relates to an exhaust purification system of an internal combustion engine.
- the hydrocarbons which are fed when releasing NO x from the NO x storage catalyst are made gaseous hydrocarbons at the oxidation catalyst, and the gaseous hydrocarbons are fed to the NO x storage catalyst.
- the NO x which is released from the NO x storage catalyst is reduced well.
- Patent Literature 1 Japanese Patent No. 3969450
- An object of the present invention is to provide an exhaust purification system of an internal combustion engine which can obtain a high NO x purification rate even if the temperature of the exhaust purification catalyst becomes a high temperature.
- an exhaust purification system of an internal combustion engine in which an exhaust purification catalyst for reacting NO x contained in exhaust gas and reformed hydrocarbons is arranged inside of an engine exhaust passage, a precious metal catalyst is carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst, the exhaust purification catalyst has a property of reducing the NO x which is contained in exhaust gas if the concentration of hydrocarbons flowing into the exhaust purification catalyst is made to change within a predetermined range of amplitude and within a predetermined range of period and has a property of being increased in storage amount of NO x which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than the predetermined range, at the time of engine operation, a main concentration changing action in which the concentration of hydrocarbons flowing into the exhaust purification catalyst is made to change within the predetermined range of amplitude and within the predetermined range of period is performed and, furthermore, before each main concentration changing action, an auxiliary
- FIG. 1 is an overall view of a compression ignition type internal combustion engine.
- FIG. 2 is a view schematically showing a surface part of a catalyst carrier.
- FIG. 3 is a view for explaining an oxidation reaction in an exhaust purification catalyst.
- FIG. 4 is a view showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.
- FIG. 5 is a view showing an NO x purification rate.
- FIG. 6A and 6B are views for explaining an oxidation reduction reaction in an exhaust purification catalyst.
- FIG. 7A and 7B are views for explaining an oxidation reduction reaction in an exhaust purification catalyst.
- FIG. 8 is a view showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.
- FIG. 9 is a view of an NO x purification rate.
- FIG. 10 is a time chart showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.
- FIG. 11 is a time chart showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.
- FIG. 12 is a view showing a relationship between an oxidizing strength of an exhaust purification catalyst and a demanded minimum air-fuel ratio X.
- FIG. 13 is a view showing a relationship between an oxygen concentration in exhaust gas and an amplitude AH of a hydrocarbon concentration giving the same NO x purification rate.
- FIG. 14 is a view showing a relationship between an amplitude ⁇ H of a hydrocarbon concentration and an NO x purification rate.
- FIG. 15 is a view showing a relationship of a vibration period ⁇ T of a hydrocarbon concentration and an NO x purification rate.
- FIGS. 16A and 16B are views showing maps of hydrogen feed amounts WA and WB.
- FIGS. 17A , 17 B, and 17 C are views showing the feed period ⁇ TA of the hydrocarbons etc. at the time of a main concentration changing action.
- FIG. 18 is a view showing a change in the air-fuel ratio of the exhaust gas flowing to the exhaust purification catalyst etc.
- FIG. 19 is a view showing a map of an exhausted NO x amount NOXA.
- FIG. 21 is a view showing a map of a hydrocarbon feed amount WR.
- FIG. 22 is a view showing an NO x purification rate and NO x storage rate.
- FIG. 23 is a flow chart for NO x purification control.
- FIG. 1 is an overall view of a compression ignition type internal combustion engine.
- 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically controlled fuel injector for injecting fuel into each combustion chamber 2 , 4 an intake manifold, and 5 an exhaust manifold.
- the intake manifold 4 is connected through an intake duct 6 to an outlet of a compressor 7 a of an exhaust turbocharger 7 , while an inlet of the compressor 7 a is connected through an intake air amount detector 8 to an air cleaner 9 .
- a throttle valve 10 driven by a step motor is arranged inside the intake duct 6 .
- a cooling device 11 is arranged for cooling the intake air which flows through the inside of the intake duct 6 .
- the engine cooling water is guided to the inside of the cooling device 11 where the engine cooling water is used to cool the intake air.
- the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7 b of the exhaust turbocharger 7 .
- the outlet of the exhaust turbine 7 b is connected through an exhaust pipe 12 to an inlet of the exhaust purification catalyst 13 , while the outlet of the exhaust purification catalyst 13 is connected to a particulate filter 14 for trapping particulate which is contained in the exhaust gas.
- a hydrocarbon feed valve 15 is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown in FIG. 1 , diesel oil is used as the hydrocarbons which are fed from the hydrocarbon feed valve 15 .
- the present invention can also be applied to a spark ignition type internal combustion engine in which fuel is burned under a lean air-fuel ratio.
- hydrocarbons comprised of gasoline or other fuel used as fuel of a spark ignition type internal combustion engine are fed.
- the exhaust manifold 5 and the intake manifold 4 are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage 16 .
- EGR exhaust gas recirculation
- an electronically controlled EGR control valve 17 is arranged inside the EGR passage 16 .
- a cooling device 18 is arranged for cooling EGR gas flowing through the inside of the EGR passage 16 .
- the engine cooling water is guided to the inside of the cooling device 18 where the engine cooling water is used to cool the EGR gas.
- each fuel injector 3 is connected through a fuel feed tube 19 to a common rail 20 .
- This common rail 20 is connected through an electronically controlled variable discharge fuel pump 21 to a fuel tank 22 .
- the fuel which is stored inside of the fuel tank 22 is fed by the fuel pump 21 to the inside of the common rail 20 .
- the fuel which is fed to the inside of the common rail 20 is fed through each fuel feed tube 19 to the fuel injector 3 .
- An electronic control unit 30 is comprised of a digital computer provided with a ROM (read only memory) 32 , a RAM (random access memory) 33 , a CPU (microprocessor) 34 , an input port 35 , and an output port 36 , which are connected with each other by a bidirectional bus 31 .
- a temperature sensor 23 Downstream of the exhaust purification catalyst 13 , a temperature sensor 23 for detecting the temperature of the exhaust purification catalyst 13 is attached.
- a differential pressure sensor 24 for detecting a differential pressure before and after the particulate filter 14 is attached.
- an air-fuel ratio sensor 25 is arranged at the collecting portion of the exhaust manifold 5 .
- the output signals of these temperature sensor 23 , differential pressure sensor 24 , the air-fuel ratio sensor 25 , and intake air amount detector 8 are input through respectively corresponding AD converters 37 to the input port 35 .
- an accelerator pedal 40 has a load sensor 41 connected to it which generates an output voltage proportional to the amount of depression L of the accelerator pedal 40 .
- the output voltage of the load sensor 41 is input through a corresponding AD converter 37 to the input port 35 .
- a crank angle sensor 42 is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°.
- the output port 36 is connected through corresponding drive circuits 38 to each fuel injector 3 , a step motor for driving the throttle valve 10 , hydrocarbon feed valve 15 , EGR control valve 17 , and fuel pump 21 .
- FIG. 2 schematically shows a surface part of a catalyst carrier which is carried on a substrate of the exhaust purification catalyst 13 .
- a catalyst carrier 50 made of alumina on which precious metal catalysts 51 and 52 are carried.
- a basic layer 53 is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanoid or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal able to donate electrons to NO x .
- the exhaust gas flows along the top of the catalyst carrier 50 , so the precious metal catalysts 51 and 52 can be said to be carried on the exhaust gas flow surface of the exhaust purification catalyst 13 .
- the surface of the basic layer 53 exhibits basicity, so the surface of the basic layer 53 is called the basic exhaust gas flow surface part 54 .
- the precious metal catalyst 51 is comprised of platinum Pt
- the precious metal catalyst 52 is comprised of rhodium Rh. That is, the precious metal catalysts 51 and 52 which are carried on the catalyst carrier 50 are comprised of platinum Pt and rhodium Rh.
- palladium Pd may be further carried or, instead of rhodium Rh, palladium Pd may be carried. That is, the precious metal catalysts 51 and 52 which are carried on the catalyst carrier 50 are comprised of platinum Pt and at least one of rhodium Rh and palladium Pd.
- FIG. 3 schematically shows the reforming action performed at the exhaust purification catalyst 13 at this time.
- the hydrocarbons HC which are injected from the hydrocarbon feed valve 15 become radical hydrocarbons HC with a small carbon number by the catalyst 51 .
- FIG. 4 shows feed timings of auxiliary hydrocarbons and main hydrocarbons from the hydrocarbon feed valve 15 and the change in the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust purification catalyst 13 .
- the change in the air-fuel ratio (A/F)in depends on the change in concentration of the hydrocarbons in the exhaust gas which flows into the exhaust purification catalyst 13 , so it can be said that the change in the air-fuel ratio (A/F)in shown in FIG. 4 expresses the change in concentration of the hydrocarbons.
- the hydrocarbon concentration becomes higher, the air-fuel ratio (A/F)in becomes smaller, so, in FIG. 4 , the more to the rich side the air-fuel ratio (A/F)in becomes, the higher the hydrocarbon concentration.
- FIG. 5 shows the NO x purification rate by the exhaust purification catalyst 13 with respect to the catalyst temperatures of the exhaust purification catalyst 13 when periodically making the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 change so as to, as shown in FIG. 4 , make the air-fuel ratio (A/F)in of the exhaust gas flowing to the exhaust purification catalyst 13 change.
- the inventors engaged in research relating to NO x purification for a long time. In the process of research, they learned that if making the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 vibrate by within a predetermined range of amplitude and within a predetermined range of period, as shown in FIG. 5 , an extremely high NO x purification rate is obtained even in a 400° C. or higher high temperature region.
- FIGS. 6A and 6B schematically show the surface part of the catalyst carrier 50 of the exhaust purification catalyst 13 .
- FIGS. 6A and 6B show the reactions believed to occur when the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 is made to vibrate within a predetermined range of amplitude and within a predetermined range of period.
- FIG. 6A shows when the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 is low
- FIG. 6B shows when the main hydrocarbons are fed from the hydrocarbon feed valve 15 and the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 becomes higher.
- the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst 13 is maintained lean except for an instant, so the exhaust gas which flows into the exhaust purification catalyst 13 normally becomes a state of oxygen excess. Therefore, the NO x which is contained in the exhaust gas, as shown in FIG. 6A , is oxidized on the platinum 51 and becomes NO 2 . Next, this NO 2 is supplied with electrons from the platinum 51 and becomes NO 2 ⁇ . Therefore, a large amount of NO 2 ⁇ is produced on the platinum 51 . This NO 2 ⁇ is strong in activity. Above, this NO 2 ⁇ is called the active NO 2 * .
- the main hydrocarbons are fed from the hydrocarbon feed valve 15 , as shown in FIG. 3 , the main hydrocarbons are reformed inside the exhaust purification catalyst 13 and become radicalized. As a result, as shown in FIG. 6B , the hydrocarbon concentration around the active NO 2 * becomes higher.
- the active NO 2 * is produced, if the state of a high oxygen concentration around the active NO 2 * continues for a predetermined time or more, the active NO 2 * is oxidized and is absorbed in the basic layer 53 in the form of nitrate ions NO 3 ⁇ .
- the active NO 2 * reacts on the platinum 51 with the radical hydrocarbons HC whereby a reducing intermediate is produced. This reducing intermediate is adhered or adsorbed on the surface of the basic layer 53 .
- the first produced reducing intermediate is considered to be a nitro compound R—NO 2 . If this nitro compound R—NO 2 is produced, the result becomes a nitrile compound R—CN, but this nitrile compound R—CN can only survive for an instant in this state, so immediately becomes an isocyanate compound R—NCO.
- This isocyanate compound R—NCO when hydrolyzed, becomes an amine compound R—NH 2 . However, in this case, what is hydrolyzed is considered to be part of the isocyanate compound R—NCO. Therefore, as shown in FIG. 6B , the majority of the reducing intermediate which is held or adsorbed on the surface of the basic layer 53 is believed to be the isocyanate compound R—NCO and amine compound R—NH 2 .
- the exhaust purification catalyst 13 by making the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 higher, a reducing intermediate is produced, while by making the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 lower to raise the oxygen concentration, the active NO 2 * reacts with the reducing intermediate where the NO x is removed. That is, to use the exhaust purification catalyst 13 to remove NO x , it is necessary to periodically change the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 .
- precious metal catalysts 51 and 52 are carried on the exhaust gas flow surface of the exhaust purification catalyst 13 .
- a basic exhaust gas flow surface part 54 is formed around the precious metal catalysts 51 and 52 .
- NO x is reduced by the reducing action of the reducing intermediate R—NCO or R—NH 2 held on the basic exhaust gas flow surface 54 and, the changing period of the main hydrocarbon concentration is made a changing period required for continued production of the reducing intermediate R—NCO or R—NH 2 .
- the injection interval is made 3 seconds.
- the reducing intermediate R—NCO or R—NH 2 disappears from the surface of the basic layer 53 .
- the active NO 2 * which was produced on the platinum Pt 53 diffuses in the basic layer 53 in the form of nitrate ions NO 3 ⁇ and becomes nitrates. That is, at this time, the NO x in the exhaust gas is absorbed in the form of nitrates inside of the basic layer 53 .
- FIG. 7B shows the case where the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst 13 is made the stoichiometric air-fuel ratio or rich when the NO x is absorbed in the form of nitrates inside of the basic layer 53 .
- the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO 3 ⁇ ⁇ NO 2 ), and consequently the nitrates absorbed in the basic layer 53 become nitrate ions NO 3 ⁇ one by one and, as shown in FIG. 7B , are released from the basic layer 53 in the form of NO 2 .
- the released NO 2 is reduced by the hydrocarbons HC and CO contained in the exhaust gas.
- FIG. 8 shows the case of making the air-fuel ratio (A/F)in of the exhaust gas which flows into the exhaust purification catalyst 13 temporarily rich slightly before the NO x absorption ability of the basic layer 53 becomes saturated.
- the time interval of this rich control is 1 minute or more.
- the NO x which was absorbed in the basic layer 53 when the air-fuel ratio (A/F)in of the exhaust gas was lean is released all at once from the basic layer 53 and reduced when the air-fuel ratio (A/F)in of the exhaust gas is made temporarily rich. Therefore, in this case, the basic layer 53 plays the role of an absorbent for temporarily absorbing NO x .
- the basic layer 53 temporarily adsorbs the NO x . Therefore, if using term of storage as a term including both absorption and adsorption, at this time, the basic layer 53 performs the role of an NO x storage agent for temporarily storing the NO x . That is, in this case, if the ratio of the air and fuel (hydrocarbons) which are supplied into the engine intake passage, combustion chambers 2 , and exhaust passage upstream of the exhaust purification catalyst 13 is referred to as the air-fuel ratio of the exhaust gas, the exhaust purification catalyst 13 functions as an NO x storage catalyst which stores the NO x when the air-fuel ratio of the exhaust gas is lean and releases the stored NO x when the oxygen concentration in the exhaust gas falls.
- FIG. 9 shows the NO x purification rate when making the exhaust purification catalyst 13 function as an NO x storage catalyst in this way.
- the abscissa of the FIG. 9 shows the catalyst temperature TC of the exhaust purification catalyst 13 .
- the catalyst temperature TC is 300° C. to 400° C.
- an extremely high NO x purification rate is obtained, but when the catalyst temperature TC becomes a 400° C. or higher high temperature, the NO x purification rate falls.
- the NO x purification rate falls because if the catalyst temperature TC becomes 400° C. or more, the nitrates break down by heat and are released in the form of NO 2 from the exhaust purification catalyst 13 . That is, so long as storing NO x in the form of nitrates, when the catalyst temperature TC is high, it is difficult to obtain a high NO x purification rate.
- the new NO x purification method shown from FIG. 4 to FIGS. 6A and 6B as will be understood from FIGS. 6A and 6B , nitrates are not formed or even if formed are extremely fine in amount, consequently, as shown in FIG. 5 , even when the catalyst temperature TC is high, a high NO x purification rate is obtained.
- an exhaust purification catalyst 13 for reacting NO x contained in exhaust gas and reformed hydrocarbons is arranged inside of an engine exhaust passage, precious metal catalysts 51 and 52 are carried on the exhaust gas flow surface of the exhaust purification catalyst 13 , a basic exhaust gas flow surface part 54 is formed around the precious metal catalysts 51 and 52 , the exhaust purification catalyst 13 has the property of reducing the NO x which is contained in exhaust gas if the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is made to change within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of NO x which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range, and, at the time of engine operation, the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 is made to change within the predetermined range of amplitude and within the predetermined range of period to thereby reduce the NO x which is contained in the exhaust gas in the exhaust purification catalyst 13
- the NO x purification method which is shown from FIG. 4 to FIGS. 6A and 6B can be said to be a new NO x purification method designed to remove NO x without forming almost any nitrates in the case of using an exhaust purification catalyst which carries a precious metal catalyst and forms a basic layer which can absorb NO x .
- this new NO x purification method when using this new NO x purification method, the nitrates which are detected from the basic layer 53 become much smaller in amount compared with the case where making the exhaust purification catalyst 13 function as an NO x storage catalyst.
- this new NO x purification method will be referred to below as the first NO x purification method.
- FIG. 10 shows enlarged the change in the air-fuel ratio (A/F)in shown in FIG. 4 .
- the change in the air-fuel ratio (A/F)in of the exhaust gas flowing into this exhaust purification catalyst 13 simultaneously shows the change in concentration of the hydrocarbons which flow into the exhaust purification catalyst 13 .
- ⁇ HA indicates the amplitude of the change in concentration of the main hydrocarbons HC which flow into the exhaust purification catalyst 13
- ⁇ TA indicates the changing period of the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 .
- (A/F)b shows the base air-fuel ratio which shows the air-fuel ratio of the combustion gas for generating the engine output.
- this base air-fuel ratio (A/F)b shows the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst 13 when stopping the feed of hydrocarbons.
- X shows the upper limit of the air-fuel ratio (A/F)in which is used for producing the reducing intermediate without the produced active NO 2 * being stored in the form of nitrates inside the basic layer 53 at the time of feed of the main hydrocarbons. To make the active NO 2 * and the reformed hydrocarbons react and produce the reducing intermediate, it is necessary to make the air-fuel ratio (A/F)in lower than the upper limit X of this air-fuel ratio.
- X shows the lower limit of the concentration of main hydrocarbons required for making the active NO 2 * and reformed main hydrocarbons react to produce a reducing intermediate.
- the concentration of main hydrocarbons has to be made higher than this lower limit X.
- whether the reducing intermediate is produced is determined by the ratio of the oxygen concentration and main hydrocarbon concentration around the active NO 2 * , that is, the air-fuel ratio (A/F)in.
- the upper limit X of the air-fuel ratio required for producing the reducing intermediate will below be called the demanded minimum air-fuel ratio.
- the demanded minimum air-fuel ratio X is rich, therefore, in this case, to form the reducing intermediate, the air-fuel ratio (A/F)in at the time of feeding the main hydrocarbons is instantaneously made the demanded minimum air-fuel ratio X or less, that is, rich.
- the demanded minimum air-fuel ratio X is lean.
- the reducing intermediate is formed by periodically reducing the air-fuel ratio (A/F)in while maintaining the air-fuel ratio (A/F)in lean.
- the exhaust purification catalyst 13 determines whether the demanded minimum air-fuel ratio X becomes rich or becomes lean depending on the oxidizing strength of the exhaust purification catalyst 13 .
- the exhaust purification catalyst 13 for example, becomes stronger in oxidizing strength if increasing the carried amount of the precious metal 51 and becomes stronger in oxidizing strength if strengthening the acidity. Therefore, the oxidizing strength of the exhaust purification catalyst 13 changes due to the carried amount of the precious metal 51 or the strength of the acidity.
- the demanded minimum air-fuel ratio X has to be reduced the stronger the oxidizing strength of the exhaust purification catalyst 13 .
- the demanded minimum air-fuel ratio X becomes lean or rich due to the oxidizing strength of the exhaust purification catalyst 13 .
- the amplitude of the change in the concentration of main hydrocarbons which flow into the exhaust purification catalyst 13 and the changing period of the concentration of the main hydrocarbons which flow into the exhaust purification catalyst 13 will be explained.
- FIG. 13 shows the relationship between the oxygen concentration in the exhaust gas before the main hydrocarbons are fed and the amplitude ⁇ HA of the main hydrocarbon concentration when the same NO x purification rate is obtained. From FIG. 13 , it is learned that to obtain the same NO x purification rate, it is necessary to increase the amplitude ⁇ HA of the main hydrocarbon concentration the higher the oxygen concentration in the exhaust gas before the main hydrocarbons are fed. That is, to obtain the same NO x purification rate, it is necessary to increase the amplitude ⁇ HA of the main hydrocarbon concentration the higher the base air-fuel ratio (A/F)b becomes. In other words, to remove the NO x well, the lower the base air-fuel ratio (A/F)b, the more the amplitude ⁇ HA of the main hydrocarbon concentration can be reduced.
- A/F base air-fuel ratio
- the changing period ⁇ TA of the main hydrocarbon concentration becomes longer, in the time after the main hydrocarbons are fed to when the main hydrocarbons are next fed, the concentration of oxygen around the active NO 2 * will become higher.
- the changing period ⁇ TA of the main hydrocarbon concentration becomes longer than about 5 seconds, the active NO 2 * starts to be absorbed in the form of nitrates in the basic layer 53 . Therefore, as shown in FIG. 15 , if the changing period ⁇ TA of main hydrocarbon concentration becomes longer than about 5 seconds, the NO x purification rate will fall. Therefore, the changing period ⁇ TA of the main hydrocarbon concentration has to be made 5 seconds or less.
- the changing period ⁇ TA of main hydrocarbon concentration becomes about 0.3 second or less, the fed main hydrocarbons will start to build up on the exhaust gas flow surface of the exhaust purification catalyst 13 . Therefore, as shown in FIG. 15 , if the changing period ⁇ TA of main hydrocarbon concentration becomes about 0.3 second or less, the NO x purification rate falls. Therefore, in the present invention, the changing period of main hydrocarbon concentration is made between 0.3 second to 5 seconds.
- the main hydrocarbon feed amount WA which enables this optimum amplitude ⁇ HA of the main hydrocarbon concentration to be obtained is stored as a function of the injection amount Q from a fuel injector 3 and engine speed N in the form of a map such as shown in FIG. 16A in advance in the ROM 32 .
- the main hydrocarbons become radical hydrocarbons with a small carbon number due to the catalyst 51 .
- the radical hydrocarbons react with the active NO 2 * on the catalyst 51 whereby the reducing intermediate R—NCO or R—NH 2 is produced.
- this reducing intermediate reacts with the active NO 2 * and becomes N 2 , CO 2 , and H 2 O.
- auxiliary hydrocarbons are fed before the feed of the main hydrocarbons. In this way, if auxiliary hydrocarbons are fed before the feed of the main hydrocarbons, the oxygen remaining at the exhaust purification catalyst 13 is consumed for oxidizing the auxiliary hydrocarbons, therefore when the main hydrocarbons are fed, the amount of oxygen remaining at the exhaust purification catalyst 13 can be reduced. As a result, a high NO x purification rate can be secured.
- the exhaust purification catalyst 13 has the property of reducing the NO x which is contained in exhaust gas if the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is made to change within the predetermined range of amplitude and within the predetermined range of period and has the property of being increased in storage amount of NO x which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range.
- the present invention as shown in FIGS.
- a main concentration changing action in which the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is made to change within the predetermined range of amplitude ⁇ HA and within the predetermined range of period ⁇ TA is performed and, furthermore, before each main concentration changing action, an auxiliary concentration changing action in which the hydrogen concentration is changed by an amplitude ⁇ HB smaller than the amplitude ⁇ HA at the time of each main concentration changing is performed.
- the auxiliary hydrocarbons are mainly meant for consuming the residual oxygen, so the feed amount of the auxiliary hydrocarbons is considerably smaller compared with the feed amount of the main hydrocarbons. Therefore, as shown in FIGS. 10 and 11 , the amplitude ⁇ HB of the hydrocarbon concentration due to the auxiliary hydrocarbons is considerably smaller than the amplitude ⁇ HA of the hydrocarbon concentration due to the main hydrocarbons.
- FIG. 13 shows the relationship between the oxygen concentration in the exhaust gas before the auxiliary hydrocarbons are fed and the amplitude ⁇ HB of the auxiliary hydrocarbons.
- the higher the oxygen concentration in the exhaust gas before the auxiliary hydrocarbons are fed the greater the amount of residual oxygen on the exhaust purification catalyst 13 . Therefore, the higher the oxygen concentration in the exhaust gas before the auxiliary hydrocarbons are fed, the greater the amplitude ⁇ HB of the auxiliary hydrocarbons is made.
- the feed period ⁇ TA of the main hydrocarbons is made shorter the higher the temperature TC of the exhaust purification catalyst 13 . That is, the period ⁇ TA when the main concentration changing action is performed is made shorter as the temperature TC of the exhaust purification catalyst 13 becomes higher.
- the feed period ⁇ TA of the main hydrocarbons is stored as a function of the injection amount Q from the fuel injector 3 and engine speed N in the form of a map shown in FIG. 17C in advance in the ROM 32 .
- the feed action of the auxiliary hydrocarbons is performed before a time ⁇ TB of the feed action of the main hydrocarbons.
- the feed of the auxiliary hydrocarbons as explained before, is mainly for consuming the residual oxygen. In this case, this time ⁇ TB does not particularly have to be changed in accordance with the engine operating state. Therefore, in this embodiment according to the present invention, the feed of the auxiliary hydrocarbons is performed before the predetermined constant time ⁇ TB when the feed of the main hydrocarbons is performed. That is, the auxiliary concentration changing action is performed before the predetermined constant time ⁇ TB when each main concentration changing action is performed.
- an NO x purification method in the case when making the exhaust purification catalyst 13 function as an NO x storage catalyst will be specifically explained.
- the NO x purification method in the case when making the exhaust purification catalyst 13 function as an NO x storage catalyst in this way will be referred to below as the second NO x purification method.
- this second NO x purification method as shown in FIG. 18 , when the stored NO x amount ⁇ NOX of NO x which is stored in the basic layer 53 has exceeds a predetermined allowable amount MAX, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust purification catalyst 13 is temporarily made rich. If the air-fuel ratio (A/F)in of the exhaust gas is made rich, the NO x which was stored in the basic layer 53 when the air-fuel ratio (A/F)in of the exhaust gas was lean is released from the basic layer 53 all at once and reduced. Due to this, the NO x is removed.
- the stored NO x amount ⁇ NOX is, for example, calculated from the amount of NO x which is exhausted from the engine.
- the exhausted NO x amount NOXA of NO x which is exhausted from the engine per unit time is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown in FIG. 19 in advance in the ROM 32 .
- the stored NO x amount ⁇ NOX is calculated from exhausted NO x amount NOXA. In this case, as explained before, the period in which the air-fuel ratio (A/F)in of the exhaust gas is made rich is usually 1 minute or more.
- FIG. 22 shows the NO x purification rate when the first NO x purification method is used for NO x purification treatment and the NO x storage rate to the exhaust purification catalyst 13 when the second NO x purification method is used.
- the NO x purification rate is higher than the NO x storage rate, that is, when the temperature TC of the exhaust purification catalyst 13 is relatively high
- the first NO x purification method is used
- the NO x storage rate is higher than the NO x purification rate, that is, when the temperature TC of the exhaust purification catalyst 13 is low
- the second NO x purification method is used. Therefore, at the time of engine startup, normally the second NO x purification method is used.
- the temperature TC of the exhaust purification catalyst 13 becomes high, the second NO x purification method is switched to the first NO x purification method.
- step 60 it is judged if the NO x storage rate to the exhaust purification catalyst 13 when the second NO x purification method is used is lower than the NO x purification rate when the first NO x purification method is used for the NO x purification treatment.
- the routine proceeds to step 61 where the second NO x purification method is performed.
- NO x amount NOXA exhausted per unit time is calculated from the map shown in FIG. 19 .
- ⁇ NOX is incremented by the exhausted NO x amount NOXA to calculate the stored NO x amount ⁇ NOX.
- the routine proceeds to step 64 where the additional fuel amount WR is calculated from the map shown in FIG. 21 and the injection action of additional fuel is performed.
- ⁇ NOX is cleared.
- step 66 the routine proceeds to step 66 where the first NO x purification method is performed. That is, at step 66 , the feed amount WA of the main hydrocarbons is calculated from the map shown in FIG. 16A , next, at step 67 , the feed amount WB of the auxiliary hydrocarbons is calculated from the map shown in FIG. 16B . Next, at step 68 , the feed period ⁇ TA of the main hydrocarbons is calculated from the map shown in FIG. 17C . Next, at step 69 , the feed of the auxiliary hydrocarbons and the main hydrocarbons is performed by using the calculated values of WA, WB ⁇ , and TA and the value of ⁇ TB.
- an oxidation catalyst for reforming the hydrocarbons in the engine exhaust passage upstream of the exhaust purification catalyst 13 , an oxidation catalyst for reforming the hydrocarbons can be arranged.
Abstract
Description
- The present invention relates to an exhaust purification system of an internal combustion engine.
- Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, an NOx storage catalyst which stores NOx which is contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich, which arranges, in the engine exhaust passage upstream of the NOx storage catalyst, an oxidation catalyst which has an adsorption function, and which feeds hydrocarbons into the engine exhaust passage upstream of the oxidation catalyst to make the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst rich when releasing NOx from the NOx storage catalyst (for example, see Patent Literature 1).
- In this internal combustion engine, the hydrocarbons which are fed when releasing NOx from the NOx storage catalyst are made gaseous hydrocarbons at the oxidation catalyst, and the gaseous hydrocarbons are fed to the NOx storage catalyst. As a result, the NOx which is released from the NOx storage catalyst is reduced well.
- Patent Literature 1: Japanese Patent No. 3969450
- However, there is the problem that when the NOx storage catalyst becomes a high temperature, the NOx purification rate falls.
- An object of the present invention is to provide an exhaust purification system of an internal combustion engine which can obtain a high NOx purification rate even if the temperature of the exhaust purification catalyst becomes a high temperature.
- According to the present invention, there is provided an exhaust purification system of an internal combustion engine in which an exhaust purification catalyst for reacting NOx contained in exhaust gas and reformed hydrocarbons is arranged inside of an engine exhaust passage, a precious metal catalyst is carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst, the exhaust purification catalyst has a property of reducing the NOx which is contained in exhaust gas if the concentration of hydrocarbons flowing into the exhaust purification catalyst is made to change within a predetermined range of amplitude and within a predetermined range of period and has a property of being increased in storage amount of NOx which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than the predetermined range, at the time of engine operation, a main concentration changing action in which the concentration of hydrocarbons flowing into the exhaust purification catalyst is made to change within the predetermined range of amplitude and within the predetermined range of period is performed and, furthermore, before each main concentration changing action, an auxiliary concentration changing action in which the concentration of hydrocarbons is made to change by an amplitude smaller than the amplitude at the time of each main concentration changing action is performed.
- Even if the temperature of the exhaust purification catalyst becomes a high temperature, a high NOx purification rate can be obtained.
-
FIG. 1 is an overall view of a compression ignition type internal combustion engine. -
FIG. 2 is a view schematically showing a surface part of a catalyst carrier. -
FIG. 3 is a view for explaining an oxidation reaction in an exhaust purification catalyst. -
FIG. 4 is a view showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst. -
FIG. 5 is a view showing an NOx purification rate. -
FIG. 6A and 6B are views for explaining an oxidation reduction reaction in an exhaust purification catalyst. -
FIG. 7A and 7B are views for explaining an oxidation reduction reaction in an exhaust purification catalyst. -
FIG. 8 is a view showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst. -
FIG. 9 is a view of an NOx purification rate. -
FIG. 10 is a time chart showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst. -
FIG. 11 is a time chart showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst. -
FIG. 12 is a view showing a relationship between an oxidizing strength of an exhaust purification catalyst and a demanded minimum air-fuel ratio X. -
FIG. 13 is a view showing a relationship between an oxygen concentration in exhaust gas and an amplitude AH of a hydrocarbon concentration giving the same NOx purification rate. -
FIG. 14 is a view showing a relationship between an amplitude ΔH of a hydrocarbon concentration and an NOx purification rate. -
FIG. 15 is a view showing a relationship of a vibration period ΔT of a hydrocarbon concentration and an NOx purification rate. -
FIGS. 16A and 16B are views showing maps of hydrogen feed amounts WA and WB. -
FIGS. 17A , 17B, and 17C are views showing the feed period ΔTA of the hydrocarbons etc. at the time of a main concentration changing action. -
FIG. 18 is a view showing a change in the air-fuel ratio of the exhaust gas flowing to the exhaust purification catalyst etc. -
FIG. 19 is a view showing a map of an exhausted NOx amount NOXA. -
FIG. 20 is a view showing a fuel injection timing. -
FIG. 21 is a view showing a map of a hydrocarbon feed amount WR. -
FIG. 22 is a view showing an NOx purification rate and NOx storage rate. -
FIG. 23 is a flow chart for NOx purification control. -
FIG. 1 is an overall view of a compression ignition type internal combustion engine. - Referring to
FIG. 1 , 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically controlled fuel injector for injecting fuel into eachcombustion chamber intake manifold 4 is connected through anintake duct 6 to an outlet of acompressor 7 a of anexhaust turbocharger 7, while an inlet of thecompressor 7 a is connected through an intake air amount detector 8 to anair cleaner 9. Inside theintake duct 6, athrottle valve 10 driven by a step motor is arranged. Furthermore, around theintake duct 6, acooling device 11 is arranged for cooling the intake air which flows through the inside of theintake duct 6. In the embodiment shown inFIG. 1 , the engine cooling water is guided to the inside of thecooling device 11 where the engine cooling water is used to cool the intake air. - On the other hand, the
exhaust manifold 5 is connected to an inlet of anexhaust turbine 7 b of theexhaust turbocharger 7. The outlet of theexhaust turbine 7 b is connected through anexhaust pipe 12 to an inlet of theexhaust purification catalyst 13, while the outlet of theexhaust purification catalyst 13 is connected to aparticulate filter 14 for trapping particulate which is contained in the exhaust gas. Inside theexhaust pipe 12 upstream of theexhaust purification catalyst 13, ahydrocarbon feed valve 15 is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown inFIG. 1 , diesel oil is used as the hydrocarbons which are fed from thehydrocarbon feed valve 15. Note that, the present invention can also be applied to a spark ignition type internal combustion engine in which fuel is burned under a lean air-fuel ratio. In this case, from thehydrocarbon feed valve 15, hydrocarbons comprised of gasoline or other fuel used as fuel of a spark ignition type internal combustion engine are fed. - On the other hand, the
exhaust manifold 5 and theintake manifold 4 are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”)passage 16. Inside theEGR passage 16, an electronically controlledEGR control valve 17 is arranged. Further, around the EGRpassage 16, acooling device 18 is arranged for cooling EGR gas flowing through the inside of the EGRpassage 16. In the embodiment shown inFIG. 1 , the engine cooling water is guided to the inside of thecooling device 18 where the engine cooling water is used to cool the EGR gas. On the other hand, eachfuel injector 3 is connected through afuel feed tube 19 to acommon rail 20. Thiscommon rail 20 is connected through an electronically controlled variabledischarge fuel pump 21 to afuel tank 22. The fuel which is stored inside of thefuel tank 22 is fed by thefuel pump 21 to the inside of thecommon rail 20. The fuel which is fed to the inside of thecommon rail 20 is fed through eachfuel feed tube 19 to thefuel injector 3. - An
electronic control unit 30 is comprised of a digital computer provided with a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, aninput port 35, and anoutput port 36, which are connected with each other by abidirectional bus 31. Downstream of theexhaust purification catalyst 13, atemperature sensor 23 for detecting the temperature of theexhaust purification catalyst 13 is attached. At theparticulate filter 14, adifferential pressure sensor 24 for detecting a differential pressure before and after theparticulate filter 14 is attached. Further, at the collecting portion of theexhaust manifold 5, an air-fuel ratio sensor 25 is arranged. The output signals of thesetemperature sensor 23,differential pressure sensor 24, the air-fuel ratio sensor 25, and intake air amount detector 8 are input through respectively correspondingAD converters 37 to theinput port 35. Further, anaccelerator pedal 40 has aload sensor 41 connected to it which generates an output voltage proportional to the amount of depression L of theaccelerator pedal 40. The output voltage of theload sensor 41 is input through acorresponding AD converter 37 to theinput port 35. Furthermore, at theinput port 35, acrank angle sensor 42 is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°. On the other hand, theoutput port 36 is connected throughcorresponding drive circuits 38 to eachfuel injector 3, a step motor for driving thethrottle valve 10,hydrocarbon feed valve 15,EGR control valve 17, andfuel pump 21. -
FIG. 2 schematically shows a surface part of a catalyst carrier which is carried on a substrate of theexhaust purification catalyst 13. At thisexhaust purification catalyst 13, as shown inFIG. 2 , for example, there is provided acatalyst carrier 50 made of alumina on whichprecious metal catalysts catalyst carrier 50, abasic layer 53 is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanoid or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal able to donate electrons to NOx. The exhaust gas flows along the top of thecatalyst carrier 50, so theprecious metal catalysts exhaust purification catalyst 13. Further, the surface of thebasic layer 53 exhibits basicity, so the surface of thebasic layer 53 is called the basic exhaust gasflow surface part 54. - On the other hand, in
FIG. 2 , theprecious metal catalyst 51 is comprised of platinum Pt, while theprecious metal catalyst 52 is comprised of rhodium Rh. That is, theprecious metal catalysts catalyst carrier 50 are comprised of platinum Pt and rhodium Rh. Note that, on thecatalyst carrier 50 of theexhaust purification catalyst 13, in addition to platinum Pt and rhodium Rh, palladium Pd may be further carried or, instead of rhodium Rh, palladium Pd may be carried. That is, theprecious metal catalysts catalyst carrier 50 are comprised of platinum Pt and at least one of rhodium Rh and palladium Pd. - If hydrocarbons are injected from the
hydrocarbon feed valve 15 into the exhaust gas, the hydrocarbons are reformed by theexhaust purification catalyst 13. In the present invention, at this time, the reformed hydrocarbons are used to remove the NOx at theexhaust purification catalyst 13.FIG. 3 schematically shows the reforming action performed at theexhaust purification catalyst 13 at this time. As shown inFIG. 3 , the hydrocarbons HC which are injected from thehydrocarbon feed valve 15 become radical hydrocarbons HC with a small carbon number by thecatalyst 51. - Note that, even if the
fuel injector 3 injects fuel, that is, hydrocarbons, into thecombustion chamber 2 in the second half of the expansion stroke or exhaust stroke, the hydrocarbons are reformed in thecombustion chamber 2 orexhaust purification catalyst 13, and the NOx which is contained in exhaust gas is removed by the reformed hydrocarbons in theexhaust purification catalyst 13. Therefore, in the present invention, instead of feeding hydrocarbons from thehydrocarbon feed valve 15 to the inside of an engine exhaust passage, it is also possible to feed hydrocarbons into thecombustion chamber 2 in the second half of the expansion stroke or exhaust stroke. In this way, in the present invention, it is possible to feed hydrocarbons into thecombustion chamber 2, but below, the present invention will be explained with reference to the case of trying to inject hydrocarbons from ahydrocarbon feed valve 15 to the inside of an engine exhaust passage -
FIG. 4 shows feed timings of auxiliary hydrocarbons and main hydrocarbons from thehydrocarbon feed valve 15 and the change in the air-fuel ratio (A/F)in of the exhaust gas flowing into theexhaust purification catalyst 13. Note that, the change in the air-fuel ratio (A/F)in depends on the change in concentration of the hydrocarbons in the exhaust gas which flows into theexhaust purification catalyst 13, so it can be said that the change in the air-fuel ratio (A/F)in shown inFIG. 4 expresses the change in concentration of the hydrocarbons. However, if the hydrocarbon concentration becomes higher, the air-fuel ratio (A/F)in becomes smaller, so, inFIG. 4 , the more to the rich side the air-fuel ratio (A/F)in becomes, the higher the hydrocarbon concentration. -
FIG. 5 shows the NOx purification rate by theexhaust purification catalyst 13 with respect to the catalyst temperatures of theexhaust purification catalyst 13 when periodically making the concentration of hydrocarbons flowing into theexhaust purification catalyst 13 change so as to, as shown inFIG. 4 , make the air-fuel ratio (A/F)in of the exhaust gas flowing to theexhaust purification catalyst 13 change. The inventors engaged in research relating to NOx purification for a long time. In the process of research, they learned that if making the concentration of hydrocarbons flowing into theexhaust purification catalyst 13 vibrate by within a predetermined range of amplitude and within a predetermined range of period, as shown inFIG. 5 , an extremely high NOx purification rate is obtained even in a 400° C. or higher high temperature region. - Furthermore, at this time, a large amount of reducing intermediate containing nitrogen and hydrocarbons continues to be held or adsorbed on the surface of the
basic layer 53, that is, on the basic exhaust gasflow surface part 54 of theexhaust purification catalyst 13. It is learned that this reducing intermediate plays a central role in obtaining a high NOx purification rate. In this case, the main hydrocarbons play a central role in the production of the reducing intermediate, while the auxiliary hydrocarbons play an auxiliary role for the production of the intermediate. Therefore, first, referring toFIGS. 6A and 6B , the NOx purification action by the main hydrocarbons will be explained. Note that, theseFIGS. 6A and 6B schematically show the surface part of thecatalyst carrier 50 of theexhaust purification catalyst 13. TheseFIGS. 6A and 6B show the reactions believed to occur when the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 is made to vibrate within a predetermined range of amplitude and within a predetermined range of period. -
FIG. 6A shows when the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 is low, whileFIG. 6B shows when the main hydrocarbons are fed from thehydrocarbon feed valve 15 and the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 becomes higher. - Now, as will be understood from
FIG. 4 , the air-fuel ratio of the exhaust gas which flows into theexhaust purification catalyst 13 is maintained lean except for an instant, so the exhaust gas which flows into theexhaust purification catalyst 13 normally becomes a state of oxygen excess. Therefore, the NOx which is contained in the exhaust gas, as shown inFIG. 6A , is oxidized on theplatinum 51 and becomes NO2. Next, this NO2 is supplied with electrons from theplatinum 51 and becomes NO2 −. Therefore, a large amount of NO2 − is produced on theplatinum 51. This NO2 − is strong in activity. Above, this NO2 − is called the active NO2 *. - On the other hand, if the main hydrocarbons are fed from the
hydrocarbon feed valve 15, as shown inFIG. 3 , the main hydrocarbons are reformed inside theexhaust purification catalyst 13 and become radicalized. As a result, as shown inFIG. 6B , the hydrocarbon concentration around the active NO2 * becomes higher. In this regard, after the active NO2 * is produced, if the state of a high oxygen concentration around the active NO2 * continues for a predetermined time or more, the active NO2 * is oxidized and is absorbed in thebasic layer 53 in the form of nitrate ions NO3 −. However, if the hydrocarbon concentration around the active NO2 * is made higher before this predetermined time passes, as shown inFIG. 6B , the active NO2 * reacts on theplatinum 51 with the radical hydrocarbons HC whereby a reducing intermediate is produced. This reducing intermediate is adhered or adsorbed on the surface of thebasic layer 53. - Note that, at this time, the first produced reducing intermediate is considered to be a nitro compound R—NO2. If this nitro compound R—NO2 is produced, the result becomes a nitrile compound R—CN, but this nitrile compound R—CN can only survive for an instant in this state, so immediately becomes an isocyanate compound R—NCO. This isocyanate compound R—NCO, when hydrolyzed, becomes an amine compound R—NH2. However, in this case, what is hydrolyzed is considered to be part of the isocyanate compound R—NCO. Therefore, as shown in
FIG. 6B , the majority of the reducing intermediate which is held or adsorbed on the surface of thebasic layer 53 is believed to be the isocyanate compound R—NCO and amine compound R—NH2. - On the other hand, as shown in
FIG. 6B , if the produced reducing intermediate is surrounded by the hydrocarbons HC, the reducing intermediate is blocked by the hydrocarbons HO and the reaction will not proceed any further. In this case, if the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 is made to fall and, due to this, the concentration of oxygen becomes higher, the hydrocarbons around the reducing intermediate will be oxidized. As a result, as shown inFIG. 6A , the reducing intermediate and the active NO2 * will react. At this time, the active NO2 * reacts with the reducing intermediate R—NCO or R—NH2 to form N2, CO2, and H2O and consequently the NOx is removed. - In this way, in the
exhaust purification catalyst 13, by making the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 higher, a reducing intermediate is produced, while by making the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 lower to raise the oxygen concentration, the active NO2 * reacts with the reducing intermediate where the NOx is removed. That is, to use theexhaust purification catalyst 13 to remove NOx, it is necessary to periodically change the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13. - Of course, in this case, it is necessary to raise the concentration of main hydrocarbons to a concentration sufficiently high for producing the reducing intermediate, while it is necessary to lower the concentration of main hydrocarbons to a concentration sufficiently lower for causing the produced reducing intermediate to react with the active NO2 *. That is, it is necessary to change the concentration of main hydrocarbons which flow into the
exhaust purification catalyst 13 within a predetermined range of amplitude. Note that, in this case, a sufficient amount of reducing intermediate R—NCO or R—NH2 has to be held on thebasic layer 53, that is, on the basic exhaust gasflow surface part 24, until the produced reducing intermediate reacts with the active NO2 *. For this reason, the basic exhaust gasflow surface part 24 is provided. - On the other hand, if lengthening the feed period of the main hydrocarbons, in the time from when the main hydrocarbons are fed to when next the main hydrocarbons are fed, the time during which the oxygen concentration becomes high will become longer. Therefore, the active NO2 * will be absorbed in the
basic layer 53 in the form of nitrates without producing a reducing intermediate. To avoid this, it is necessary to make the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 change by a predetermined range of period. - Therefore, in an embodiment of the present invention, to make the NOx which is contained in the exhaust gas and the reformed hydrocarbons react and produce the reducing intermediate R—NCO or R—NH2 containing nitrogen and hydrocarbons,
precious metal catalysts exhaust purification catalyst 13. To hold the produced reducing intermediate R—NCO or R—NH2 inside theexhaust purification catalyst 13, a basic exhaust gasflow surface part 54 is formed around theprecious metal catalysts gas flow surface 54 and, the changing period of the main hydrocarbon concentration is made a changing period required for continued production of the reducing intermediate R—NCO or R—NH2. Incidentally, in the example shown inFIG. 4 , the injection interval is made 3 seconds. - If making the changing period of main hydrocarbon concentration, that is, the feed period of the main hydrocarbons HC, longer than the above predetermined range of period, the reducing intermediate R—NCO or R—NH2 disappears from the surface of the
basic layer 53. At this time, the active NO2 * which was produced on theplatinum Pt 53, as shown inFIG. 7A , diffuses in thebasic layer 53 in the form of nitrate ions NO3 − and becomes nitrates. That is, at this time, the NOx in the exhaust gas is absorbed in the form of nitrates inside of thebasic layer 53. - On the other hand,
FIG. 7B shows the case where the air-fuel ratio of the exhaust gas which flows into theexhaust purification catalyst 13 is made the stoichiometric air-fuel ratio or rich when the NOx is absorbed in the form of nitrates inside of thebasic layer 53. In this case, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO3 −→NO2), and consequently the nitrates absorbed in thebasic layer 53 become nitrate ions NO3 − one by one and, as shown inFIG. 7B , are released from thebasic layer 53 in the form of NO2. Next, the released NO2 is reduced by the hydrocarbons HC and CO contained in the exhaust gas. -
FIG. 8 shows the case of making the air-fuel ratio (A/F)in of the exhaust gas which flows into theexhaust purification catalyst 13 temporarily rich slightly before the NOx absorption ability of thebasic layer 53 becomes saturated. Note that, in the example shown inFIG. 8 , the time interval of this rich control is 1 minute or more. In this case, the NOx which was absorbed in thebasic layer 53 when the air-fuel ratio (A/F)in of the exhaust gas was lean is released all at once from thebasic layer 53 and reduced when the air-fuel ratio (A/F)in of the exhaust gas is made temporarily rich. Therefore, in this case, thebasic layer 53 plays the role of an absorbent for temporarily absorbing NOx. - Note that, at this time, sometimes the
basic layer 53 temporarily adsorbs the NOx. Therefore, if using term of storage as a term including both absorption and adsorption, at this time, thebasic layer 53 performs the role of an NOx storage agent for temporarily storing the NOx. That is, in this case, if the ratio of the air and fuel (hydrocarbons) which are supplied into the engine intake passage,combustion chambers 2, and exhaust passage upstream of theexhaust purification catalyst 13 is referred to as the air-fuel ratio of the exhaust gas, theexhaust purification catalyst 13 functions as an NOx storage catalyst which stores the NOx when the air-fuel ratio of the exhaust gas is lean and releases the stored NOx when the oxygen concentration in the exhaust gas falls. -
FIG. 9 shows the NOx purification rate when making theexhaust purification catalyst 13 function as an NOx storage catalyst in this way. Note that, the abscissa of theFIG. 9 shows the catalyst temperature TC of theexhaust purification catalyst 13. When making theexhaust purification catalyst 13 function as an NOx storage catalyst, as shown inFIG. 9 , when the catalyst temperature TC is 300° C. to 400° C., an extremely high NOx purification rate is obtained, but when the catalyst temperature TC becomes a 400° C. or higher high temperature, the NOx purification rate falls. - In this way, when the catalyst temperature TC becomes 400° C. or more, the NOx purification rate falls because if the catalyst temperature TC becomes 400° C. or more, the nitrates break down by heat and are released in the form of NO2 from the
exhaust purification catalyst 13. That is, so long as storing NOx in the form of nitrates, when the catalyst temperature TC is high, it is difficult to obtain a high NOx purification rate. However, in the new NOx purification method shown fromFIG. 4 toFIGS. 6A and 6B , as will be understood fromFIGS. 6A and 6B , nitrates are not formed or even if formed are extremely fine in amount, consequently, as shown inFIG. 5 , even when the catalyst temperature TC is high, a high NOx purification rate is obtained. - Therefore, in the present invention, an
exhaust purification catalyst 13 for reacting NOx contained in exhaust gas and reformed hydrocarbons is arranged inside of an engine exhaust passage,precious metal catalysts exhaust purification catalyst 13, a basic exhaust gasflow surface part 54 is formed around theprecious metal catalysts exhaust purification catalyst 13 has the property of reducing the NOx which is contained in exhaust gas if the concentration of hydrocarbons flowing into theexhaust purification catalyst 13 is made to change within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of NOx which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range, and, at the time of engine operation, the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 is made to change within the predetermined range of amplitude and within the predetermined range of period to thereby reduce the NOx which is contained in the exhaust gas in theexhaust purification catalyst 13. - That is, the NOx purification method which is shown from
FIG. 4 toFIGS. 6A and 6B can be said to be a new NOx purification method designed to remove NOx without forming almost any nitrates in the case of using an exhaust purification catalyst which carries a precious metal catalyst and forms a basic layer which can absorb NOx. In actuality, when using this new NOx purification method, the nitrates which are detected from thebasic layer 53 become much smaller in amount compared with the case where making theexhaust purification catalyst 13 function as an NOx storage catalyst. Note that, this new NOx purification method will be referred to below as the first NOx purification method. - Next, referring to
FIG. 10 toFIG. 15 , this first NOx purification method will be explained in a bit more detail. -
FIG. 10 shows enlarged the change in the air-fuel ratio (A/F)in shown inFIG. 4 . Note that, as explained above, the change in the air-fuel ratio (A/F)in of the exhaust gas flowing into thisexhaust purification catalyst 13 simultaneously shows the change in concentration of the hydrocarbons which flow into theexhaust purification catalyst 13. Note that, inFIG. 10 , ΔHA indicates the amplitude of the change in concentration of the main hydrocarbons HC which flow into theexhaust purification catalyst 13, while ΔTA indicates the changing period of the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13. - Furthermore, in
FIG. 10 , (A/F)b shows the base air-fuel ratio which shows the air-fuel ratio of the combustion gas for generating the engine output. In other words, this base air-fuel ratio (A/F)b shows the air-fuel ratio of the exhaust gas which flows into theexhaust purification catalyst 13 when stopping the feed of hydrocarbons. On the other hand, inFIG. 10 , X shows the upper limit of the air-fuel ratio (A/F)in which is used for producing the reducing intermediate without the produced active NO2 * being stored in the form of nitrates inside thebasic layer 53 at the time of feed of the main hydrocarbons. To make the active NO2 * and the reformed hydrocarbons react and produce the reducing intermediate, it is necessary to make the air-fuel ratio (A/F)in lower than the upper limit X of this air-fuel ratio. - In other words, in
FIG. 10 , X shows the lower limit of the concentration of main hydrocarbons required for making the active NO2 * and reformed main hydrocarbons react to produce a reducing intermediate. To produce the reducing intermediate, the concentration of main hydrocarbons has to be made higher than this lower limit X. In this case, whether the reducing intermediate is produced is determined by the ratio of the oxygen concentration and main hydrocarbon concentration around the active NO2 *, that is, the air-fuel ratio (A/F)in. The upper limit X of the air-fuel ratio required for producing the reducing intermediate will below be called the demanded minimum air-fuel ratio. - In the example shown in
FIG. 10 , the demanded minimum air-fuel ratio X is rich, therefore, in this case, to form the reducing intermediate, the air-fuel ratio (A/F)in at the time of feeding the main hydrocarbons is instantaneously made the demanded minimum air-fuel ratio X or less, that is, rich. As opposed to this, in the example shown inFIG. 11 , the demanded minimum air-fuel ratio X is lean. In this case, at the time of feeding the main hydrocarbons, the reducing intermediate is formed by periodically reducing the air-fuel ratio (A/F)in while maintaining the air-fuel ratio (A/F)in lean. - In this regard, whether the demanded minimum air-fuel ratio X becomes rich or becomes lean depends on the oxidizing strength of the
exhaust purification catalyst 13. In this case, theexhaust purification catalyst 13, for example, becomes stronger in oxidizing strength if increasing the carried amount of theprecious metal 51 and becomes stronger in oxidizing strength if strengthening the acidity. Therefore, the oxidizing strength of theexhaust purification catalyst 13 changes due to the carried amount of theprecious metal 51 or the strength of the acidity. - Now, when using an
exhaust purification catalyst 13 with a strong oxidizing strength, as shown inFIG. 11 , if maintaining the air-fuel ratio (A/F)in lean while periodically making the air-fuel ratio (A/F)in drop at the time of feed of the main hydrocarbons, when the air-fuel ratio (A/F)in is made to drop, the main hydrocarbons end up being completely oxidized and, as a result, the reducing intermediate can no longer be produced. As opposed to this, when using anexhaust purification catalyst 13 with a weak oxidizing strength, as shown inFIG. 10 , if periodically making the air-fuel ratio (A/F)in at the time of feed of the main hydrocarbons rich, when the air-fuel ratio (A/F)in is made rich, the main hydrocarbons will be partially oxidized rather than being completely oxidized, that is, the main hydrocarbons will be reformed, and therefore the reducing intermediate will be produced. Therefore, when using anexhaust purification catalyst 13 with a strong oxidizing strength, the demanded minimum air-fuel ratio X has to be made rich. - On the other hand, when using an
exhaust purification catalyst 13 with a weak oxidizing strength, as shown inFIG. 11 , if maintaining the air-fuel ratio (A/F)in lean while periodically making the air-fuel ratio - (A/F)in drop at the time of feed of the main hydrocarbons, the main hydrocarbons will be partially oxidized rather than being completely oxidized, that is, the main hydrocarbons will be reformed, and therefore the reducing intermediate will be produced. As opposed to this, when using an
exhaust purification catalyst 13 with a weak oxidizing strength, as shown inFIG. 10 , if periodically making the air-fuel ratio (A/F)in at the time of feed of the main hydrocarbons rich, a large amount of the main hydrocarbons will be exhausted from theexhaust purification catalyst 13 without being oxidized, therefore the amount of wastefully consumed hydrocarbons will increase. Therefore, when using anexhaust purification catalyst 13 with a weak oxidizing strength, the demanded minimum air-fuel ratio X has to be made lean. - That is, it is learned that the demanded minimum air-fuel ratio X, as shown in
FIG. 12 , has to be reduced the stronger the oxidizing strength of theexhaust purification catalyst 13. In this way the demanded minimum air-fuel ratio X becomes lean or rich due to the oxidizing strength of theexhaust purification catalyst 13. Below, using as an example the case where the demanded minimum air-fuel ratio X is rich, the amplitude of the change in the concentration of main hydrocarbons which flow into theexhaust purification catalyst 13 and the changing period of the concentration of the main hydrocarbons which flow into theexhaust purification catalyst 13 will be explained. - Now, if the base air-fuel ratio (A/F)b becomes larger, that is, if the oxygen concentration in the exhaust gas before the main hydrocarbons are fed becomes higher, the feed amount of the main hydrocarbons necessary for making the air-fuel ratio (A/F)in the demanded minimum air-fuel ratio X or less increases and, along with this, the amount of excess hydrocarbons which do not contribute to the production of the reducing intermediate also increases. In this case, to remove the NOx well, as explained above, it is necessary to make the excess hydrocarbons oxidize. Therefore, to remove the NOx well, the larger the amount of excess hydrocarbons, the larger the amount of oxygen which is required.
- In this case, if raising the oxygen concentration in the exhaust gas, the amount of oxygen can be increased. Therefore, to remove the NOx well, when the oxygen concentration in the exhaust gas is high before the main hydrocarbons are fed, it is necessary to raise the oxygen concentration in the exhaust gas after the main hydrocarbons are fed. That is, it is necessary to increase the amplitude of the main hydrocarbon concentration the higher the oxygen concentration in the exhaust gas before the main hydrocarbons are fed.
-
FIG. 13 shows the relationship between the oxygen concentration in the exhaust gas before the main hydrocarbons are fed and the amplitude ΔHA of the main hydrocarbon concentration when the same NOx purification rate is obtained. FromFIG. 13 , it is learned that to obtain the same NOx purification rate, it is necessary to increase the amplitude ΔHA of the main hydrocarbon concentration the higher the oxygen concentration in the exhaust gas before the main hydrocarbons are fed. That is, to obtain the same NOx purification rate, it is necessary to increase the amplitude ΔHA of the main hydrocarbon concentration the higher the base air-fuel ratio (A/F)b becomes. In other words, to remove the NOx well, the lower the base air-fuel ratio (A/F)b, the more the amplitude ΔHA of the main hydrocarbon concentration can be reduced. - In this regard, the base air-fuel ratio (A/F)b becomes the lowest at the time of an acceleration operation. At this time, if the amplitude ΔHA of the main hydrocarbon concentration is about 200 ppm, it is possible to remove the NOx well. The base air-fuel ratio (A/F)b is usually larger than at the time of acceleration. Therefore, as shown in
FIG. 14 , if the amplitude ΔHA of the main hydrocarbon concentration is 200 ppm or more, a good NOx purification rate can be obtained. - On the other hand, it is learned that when the base air-fuel ratio (A/F)b is the highest, if making the amplitude ΔHA of the main hydrocarbon concentration about 10000 ppm, a good NOx purification rate is obtained. Therefore, in the present invention, the predetermined range of the amplitude ΔHA of the main hydrocarbon concentration is made 200 ppm to 10000 ppm.
- Further, if the changing period ΔTA of the main hydrocarbon concentration becomes longer, in the time after the main hydrocarbons are fed to when the main hydrocarbons are next fed, the concentration of oxygen around the active NO2 * will become higher. In this case, if the changing period ΔTA of the main hydrocarbon concentration becomes longer than about 5 seconds, the active NO2 * starts to be absorbed in the form of nitrates in the
basic layer 53. Therefore, as shown inFIG. 15 , if the changing period ΔTA of main hydrocarbon concentration becomes longer than about 5 seconds, the NOx purification rate will fall. Therefore, the changing period ΔTA of the main hydrocarbon concentration has to be made 5 seconds or less. - On the other hand, if the changing period ΔTA of main hydrocarbon concentration becomes about 0.3 second or less, the fed main hydrocarbons will start to build up on the exhaust gas flow surface of the
exhaust purification catalyst 13. Therefore, as shown inFIG. 15 , if the changing period ΔTA of main hydrocarbon concentration becomes about 0.3 second or less, the NOx purification rate falls. Therefore, in the present invention, the changing period of main hydrocarbon concentration is made between 0.3 second to 5 seconds. - Now, in the present invention, by changing the main hydrocarbon feed amount and injection timing from the
hydrocarbon feed valve 15, the amplitude ΔHA of the main hydrocarbon concentration and feed period ΔTA are controlled so as to become the optimum values in accordance with the engine operating state. In this case, in this embodiment of the present invention, the main hydrocarbon feed amount WA which enables this optimum amplitude ΔHA of the main hydrocarbon concentration to be obtained is stored as a function of the injection amount Q from afuel injector 3 and engine speed N in the form of a map such as shown inFIG. 16A in advance in theROM 32. - Next, the effect of the feed of the auxiliary hydrocarbons on the NOx purification rate will be explained.
- As explained before, if the main hydrocarbons are fed, the main hydrocarbons become radical hydrocarbons with a small carbon number due to the
catalyst 51. The radical hydrocarbons react with the active NO2 * on thecatalyst 51 whereby the reducing intermediate R—NCO or R—NH2 is produced. Next, this reducing intermediate reacts with the active NO2 * and becomes N2, CO2, and H2O. - In this regard, when the main hydrocarbons are fed, if a large amount of oxygen remains on the
exhaust purification catalyst 13, that is, at thecatalyst 51, the number of catalyst surface parts causing a reaction between the radical hydrocarbons and active NO2 * on thecatalyst 51 decreases, therefore the amount of production of the reducing intermediate falls. If the amount of production of the reducing intermediate falls, the NOx purification rate falls. In this case, the greater the amount of oxygen remaining on theexhaust purification catalyst 13, the lower the NOx purification rate. - In this way, to keep the NOx purification rate from falling, it is necessary to remove the oxygen remaining on the
exhaust purification catalyst 13 before the feed of the main hydrocarbons. Therefore, in the present invention, to remove the residual oxygen on theexhaust purification catalyst 13, auxiliary hydrocarbons are fed before the feed of the main hydrocarbons. In this way, if auxiliary hydrocarbons are fed before the feed of the main hydrocarbons, the oxygen remaining at theexhaust purification catalyst 13 is consumed for oxidizing the auxiliary hydrocarbons, therefore when the main hydrocarbons are fed, the amount of oxygen remaining at theexhaust purification catalyst 13 can be reduced. As a result, a high NOx purification rate can be secured. - That is, as explained before, the
exhaust purification catalyst 13 has the property of reducing the NOx which is contained in exhaust gas if the concentration of hydrocarbons flowing into theexhaust purification catalyst 13 is made to change within the predetermined range of amplitude and within the predetermined range of period and has the property of being increased in storage amount of NOx which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range. In this case, in the present invention, as shown inFIGS. 10 and 11 , at the time of engine operation, a main concentration changing action in which the concentration of hydrocarbons flowing into theexhaust purification catalyst 13 is made to change within the predetermined range of amplitude ΔHA and within the predetermined range of period ΔTA is performed and, furthermore, before each main concentration changing action, an auxiliary concentration changing action in which the hydrogen concentration is changed by an amplitude ΔHB smaller than the amplitude ΔHA at the time of each main concentration changing is performed. - In this case, the main concentration changing action is performed by the feed of the main hydrocarbons, while the auxiliary concentration changing action is performed by the feed of the auxiliary hydrocarbons. In this regard, the auxiliary hydrocarbons burn easier in a lean atmosphere than a rich atmosphere. Therefore, the residual oxygen on the
exhaust purification catalyst 13 is consumed easier at the time of a lean atmosphere. Therefore, in this embodiment according to the present invention, as shown inFIG. 10 andFIG. 11 , the feed of the auxiliary hydrocarbons, that is, the auxiliary concentration changing action, is performed while maintaining the air-fuel ratio of the exhaust gas flowing into theexhaust purification catalyst 13 lean. - On the other hand, the auxiliary hydrocarbons are mainly meant for consuming the residual oxygen, so the feed amount of the auxiliary hydrocarbons is considerably smaller compared with the feed amount of the main hydrocarbons. Therefore, as shown in
FIGS. 10 and 11 , the amplitude ΔHB of the hydrocarbon concentration due to the auxiliary hydrocarbons is considerably smaller than the amplitude ΔHA of the hydrocarbon concentration due to the main hydrocarbons. - On the other hand,
FIG. 13 shows the relationship between the oxygen concentration in the exhaust gas before the auxiliary hydrocarbons are fed and the amplitude ΔHB of the auxiliary hydrocarbons. In this regard, the higher the oxygen concentration in the exhaust gas before the auxiliary hydrocarbons are fed, the greater the amount of residual oxygen on theexhaust purification catalyst 13. Therefore, the higher the oxygen concentration in the exhaust gas before the auxiliary hydrocarbons are fed, the greater the amplitude ΔHB of the auxiliary hydrocarbons is made. In this embodiment according to the present invention, the hydrocarbon feed amount WB which enables the optimum amplitude ΔHB of the auxiliary hydrocarbon concentration to be obtained is stored as a function of the injection amount Q from thefuel injector 3 and engine speed N in the form of a map such as shown inFIG. 16B in advance in theROM 32. - Note that, if feeding the auxiliary hydrocarbons, the heat of oxidation reaction of the hydrocarbons causes the temperature of the
exhaust purification catalyst 13 to rise. That is, the auxiliary hydrocarbons also have the function of raising the temperature of theexhaust purification catalyst 13. - Next, referring to
FIGS. 17A to 17C , the feed period ΔTA of the main hydrocarbons and the feed period ΔTB of the auxiliary hydrocarbons will be explained. -
FIG. 17A shows the holding time of the reducing intermediate which is produced inside theexhaust purification catalyst 13. If the temperature TC of theexhaust purification catalyst 13 rises, the reducing intermediate is easily desorbed from theexhaust purification catalyst 13. Therefore, as shown inFIG. 17A , the holding time of the reducing intermediate becomes shorter as the temperature TC of theexhaust purification catalyst 13 becomes higher. In this regard, if the holding time of the reducing intermediate becomes shorter than the feed period ΔTA of the main hydrocarbons, a state will occur where there is no reducing intermediate and, as a result, the NOx purification rate will end up falling. At this time, to prevent the NOx purification rate from falling, the feed period ΔTA of the main hydrocarbons has to be made equal to the holding time of the reducing intermediate or has to be made shorter than the holding time of the reducing intermediate. - Therefore, in this embodiment according to the present invention, as shown in
FIG. 17B , the feed period ΔTA of the main hydrocarbons is made shorter the higher the temperature TC of theexhaust purification catalyst 13. That is, the period ΔTA when the main concentration changing action is performed is made shorter as the temperature TC of theexhaust purification catalyst 13 becomes higher. In this embodiment according to the present invention, the feed period ΔTA of the main hydrocarbons is stored as a function of the injection amount Q from thefuel injector 3 and engine speed N in the form of a map shown inFIG. 17C in advance in theROM 32. - On the other hand, as shown in
FIGS. 10 and 11 , the feed action of the auxiliary hydrocarbons is performed before a time ΔTB of the feed action of the main hydrocarbons. The feed of the auxiliary hydrocarbons, as explained before, is mainly for consuming the residual oxygen. In this case, this time ΔTB does not particularly have to be changed in accordance with the engine operating state. Therefore, in this embodiment according to the present invention, the feed of the auxiliary hydrocarbons is performed before the predetermined constant time ΔTB when the feed of the main hydrocarbons is performed. That is, the auxiliary concentration changing action is performed before the predetermined constant time ΔTB when each main concentration changing action is performed. - Next, referring to
FIG. 18 toFIG. 21 , an NOx purification method in the case when making theexhaust purification catalyst 13 function as an NOx storage catalyst will be specifically explained. The NOx purification method in the case when making theexhaust purification catalyst 13 function as an NOx storage catalyst in this way will be referred to below as the second NOx purification method. - In this second NOx purification method, as shown in
FIG. 18 , when the stored NOx amount ΣNOX of NOx which is stored in thebasic layer 53 has exceeds a predetermined allowable amount MAX, the air-fuel ratio (A/F)in of the exhaust gas flowing into theexhaust purification catalyst 13 is temporarily made rich. If the air-fuel ratio (A/F)in of the exhaust gas is made rich, the NOx which was stored in thebasic layer 53 when the air-fuel ratio (A/F)in of the exhaust gas was lean is released from thebasic layer 53 all at once and reduced. Due to this, the NOx is removed. - The stored NOx amount ΣNOX is, for example, calculated from the amount of NOx which is exhausted from the engine. In this embodiment according to the present invention, the exhausted NOx amount NOXA of NOx which is exhausted from the engine per unit time is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown in
FIG. 19 in advance in theROM 32. The stored NOx amount ΣNOX is calculated from exhausted NOx amount NOXA. In this case, as explained before, the period in which the air-fuel ratio (A/F)in of the exhaust gas is made rich is usually 1 minute or more. - In this second NOx purification method, as shown in
FIG. 20 , thefuel injector 3 injects additional fuel WR into thecombustion chamber 2 in addition to the combustion-use fuel Q so that the air-fuel ratio (A/F)in of the exhaust gas flowing into theexhaust purification catalyst 13 is made rich. Note that, inFIG. 20 , the abscissa indicates the crank angle. This additional fuel WR is injected at a timing at which it will burn, but will not appear as engine output, that is, slightly before ATDC90° after compression top dead center. This fuel amount WR is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown inFIG. 21 in advance in theROM 32. Of course, in this case, it is also possible to make the amount of feed of hydrocarbons from thehydrocarbon feed valve 15 increase so as to make the air-fuel ratio (A/F)in of the exhaust gas rich. - Next, NOx release control according to the present invention will be explained.
-
FIG. 22 shows the NOx purification rate when the first NOx purification method is used for NOx purification treatment and the NOx storage rate to theexhaust purification catalyst 13 when the second NOx purification method is used. In the present invention, when the NOx purification rate is higher than the NOx storage rate, that is, when the temperature TC of theexhaust purification catalyst 13 is relatively high, the first NOx purification method is used, while when the NOx storage rate is higher than the NOx purification rate, that is, when the temperature TC of theexhaust purification catalyst 13 is low, the second NOx purification method is used. Therefore, at the time of engine startup, normally the second NOx purification method is used. When the temperature TC of theexhaust purification catalyst 13 becomes high, the second NOx purification method is switched to the first NOx purification method. -
FIG. 23 shows the NOx purification control routine. This routine is executed by interruption every predetermined time. - Referring to
FIG. 23 , first, atstep 60, it is judged if the NOx storage rate to theexhaust purification catalyst 13 when the second NOx purification method is used is lower than the NOx purification rate when the first NOx purification method is used for the NOx purification treatment. When the NOx storage rate is higher than the NOx purification rate, the routine proceeds to step 61 where the second NOx purification method is performed. - That is, at
step 61, NOx amount NOXA exhausted per unit time is calculated from the map shown inFIG. 19 . Next, atstep 62, ΣNOX is incremented by the exhausted NOx amount NOXA to calculate the stored NOx amount ΣNOX. Next, atstep 63, it is judged if stored NOx amount ΣNOX exceeds the allowable value MAX. When ΣNOX>MAX, the routine proceeds to step 64 where the additional fuel amount WR is calculated from the map shown inFIG. 21 and the injection action of additional fuel is performed. Next, atstep 65, ΣNOX is cleared. - As opposed to this, when it is judged at
step 60 that the NOx purification rate is higher than the NOx storage rate, the routine proceeds to step 66 where the first NOx purification method is performed. That is, atstep 66, the feed amount WA of the main hydrocarbons is calculated from the map shown inFIG. 16A , next, atstep 67, the feed amount WB of the auxiliary hydrocarbons is calculated from the map shown inFIG. 16B . Next, atstep 68, the feed period ΔTA of the main hydrocarbons is calculated from the map shown inFIG. 17C . Next, atstep 69, the feed of the auxiliary hydrocarbons and the main hydrocarbons is performed by using the calculated values of WA, WBΔ, and TA and the value of ΔTB. - Note that, as another embodiment, in the engine exhaust passage upstream of the
exhaust purification catalyst 13, an oxidation catalyst for reforming the hydrocarbons can be arranged. -
- 4 . . . intake manifold
- 5 . . . exhaust manifold
- 7 . . . exhaust turbocharger
- 12 . . . exhaust pipe
- 13 . . . exhaust purification catalyst
- 14 . . . particulate filter
- 15 . . . hydrocarbon feed valve
Claims (8)
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Also Published As
Publication number | Publication date |
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JP5168412B2 (en) | 2013-03-21 |
EP2610450A1 (en) | 2013-07-03 |
US8656706B2 (en) | 2014-02-25 |
EP2610450B1 (en) | 2016-08-17 |
ES2600959T3 (en) | 2017-02-13 |
CN103052771A (en) | 2013-04-17 |
WO2012029189A1 (en) | 2012-03-08 |
EP2610450A4 (en) | 2014-12-10 |
CN103052771B (en) | 2015-11-25 |
JPWO2012029189A1 (en) | 2013-10-28 |
EP2610450A8 (en) | 2013-10-16 |
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